Enhancement: Skip Nav link and content edit(s)

Added in Skip Navigation link as well as including the 'form' role in the list of the basic landmark aria roles. It's not very well supported by much besides current tech, but I feel like it should be included. 

re: https://github.com/a11yproject/a11yproject.com/issues/159 & https://github.com/a11yproject/a11yproject.com/issues/160